gdb: remove add_alias_cmd overload that accepts a string

gdb: remove add_alias_cmd overload that accepts a string

Same idea as previous patch, but for add_alias_cmd.  Remove the overload
that accepts the target command as a string (the target command name),
leaving only the one that takes the cmd_list_element.

gdb/ChangeLog:

	* command.h (add_alias_cmd): Accept target as
	cmd_list_element.  Update callers.

Change-Id: I546311f411e9e7da9302322d6ffad4e6c56df266